Apparatus and method of controlling low frequency load currents drawn from a DC source in a telecommunications system

ABSTRACT

A circuit controls the low frequency load currents drawn by components of telecommunications systems. The circuit includes a power converter, a first sense circuit, a second sense circuit, a comparator, and a power converter control circuit. The power converter control circuit controls the power converter&#39;s duty cycle in accordance with the input signal compared to a reference. In this manner, the low frequency load currents may be easily and economically controlled.

BACKGROUND OF THE INVENTION

[0001] The present invention relations to a circuit for controlling lowfrequency noise currents drawn from DC power sources by loads requiringsubstantial AC energy at low frequencies.

[0002] In telecommunication systems, fans are generally used to reduceequipment's temperature rise. The fan's speed is controlled by a motor,which is usually controlled by a voltage applied across its terminal. Amotor speed controller is used to control the motor speed and maintain adesired voltage level. The motor speed controller is often requiredbecause the telecommunication system's battery voltage often varies overa wide range. For example, the voltage range of a 48V system can varyfrom 36V to 75V, with a transient up to 100V. Thus, the motor speedcontroller provides a constant voltage to the motor terminals tomaintain the constant motor speed when the battery voltage changes overa wide range.

[0003] It is well known that a direct current (DC) motor will draw acurrent that has a significant low frequency alternating current (AC)component. For example, referring to FIG. 1, a waveform is resultingfrom a DC motor with 24V/1.5 A (36 W) output is illustrated. With 24V DCvoltage applied to the motor, the DC value of the motor current is about1.5 A. However, the AC component is 2.5 A peak to peak. The frequency ofthe AC current is about 275 Hz. Additionally, there are significantharmonic components in the waveform.

[0004] The motor speed controller is a DC to DC switching power supplywith an inherently fast response for maintaining the output voltage at aconstant level. The low frequency AC component of the motor current isreflected in the input of the motor speed controller. Therefore, theinput current of the motor speed controller has a similar shape, whichincludes a significant low frequency AC component.

[0005] Existing telecommunication equipment standards require that theinput current of the motor controller must meet both wide band and voiceband noise limits, which are much lower than the AC current, describedabove. In the example illustrated in FIG. 1, the noise of the AC currentis approximately 43 dbrnc and the standard requires that the noiseshould be less than 9 dbrnc. For a 3 db noise margin, a 37 dbrnc(43−9+3) attenuation of the AC current is required.

[0006] Referring to FIG. 2, a standard method for attenuating the ACcurrent is illustrated. An inductor-capacitor (LC) low frequency filteris added between the telecommunications equipment power source and themotor speed controller. A current I_(motor) between the motor speedcontroller and the fan comprises a large low frequency AC currentcomponent. There is also a significant amount of low frequency ACcurrent in an input current I_(Q) of the motor speed controller. Thevalue of capacitor C1 and inductor L1 is very large, thus the impedanceof capacitor C1, XC1, is very small and the impedance of L1, XL1, isvery large. Therefore, a low frequency AC current I_(ac) flows throughthe capacitor C1 and a DC current I_(dc) flows through the inductor L1.As a result, the input battery current, I_(bat) is DC.

[0007] However, since the frequency of the AC current is very low,approximately 275 Hz in the above example, the values of L1 and C1should be very large. In the above example, in order to meet therequirements set by the standard, the values for L1 and C1 are selectedas C1=2200 uF and L1=1000 uH. The size of such an inductor is fairlylarge, increasing its cost and space requirement. Furthermore, althoughthe normal operating voltage for the system is 48V, the voltage ratingfor the capacitor C1 should be higher than 48V because the input voltagemay range between 36V and 75V or even wider. Such a capacitor furtheradds to the cost and space requirement for the system.

[0008] Referring to FIG. 3, an alternate solution to the problem isillustrated. In this solution, an active filter is placed between thetelecommunications equipment power source and the motor speedcontroller. An active filter controller controls the voltage acrossMOSFET Q in such a way that the terminal characteristics of the MOSFET,that is the voltage across drain to source and the current through thedrain to source, behaves like a large inductor.

[0009] However, the problem with the active filter is a large power lossin the MOSFET Q. The voltage across the MOSFET Q (VDS) should be higherthan the worst case ripple voltage across capacitor C1 in order for theactive filter to operate properly. The worst case ripple voltage happensat minimum input voltage when the input current is at maximum value.Another limitation is the gain of the MOSFET Q is very small when itoperates at low drain to source voltage region. In a practicalsituation, the voltage across MOSFET Q should be larger than 1V, and istypically between 2-3 V. This causes significant power loss in theMOSFET Q.

[0010] Further, it should be noted that in other applications, it isrequired to attenuate low frequency noise generated by electronic loadsother than a motor of a fan. Such loads present the same difficulties asthat described for fan.

[0011] It is an object of the present invention to obviate or mitigateat least some of the above mentioned disadvantages.

BRIEF SUMMARY OF THE INVENTION

[0012] In accordance with an aspect of the present invention, there isprovided a circuit for reducing low frequency noise currents drawn froma DC power supply by a load circuit requiring AC power at lowfrequencies. The circuit comprises the following components. A powerconverter converts an input voltage to a desired output voltage. A firstsense circuit senses an input current signal to said power converter. Asecond sense circuit senses a voltage signal at the load. A comparatoris coupled with the second sense circuit for determining, and providingas its output, a signal representative of a difference between thesignal at the load and a predefined reference voltage. The comparatorhas a limited bandwidth as compared to the AC load requirement. A powerconverter control circuit has its inputs coupled with the first sensecircuit and the comparator output, and its output is coupled with thepower converter for controlling a duty cycle of the switch. The dutycycle is controlled in accordance with a relationship between the inputsignal and the comparator output.

[0013] The present invention relates to a load voltage controller thatinhibits low frequency AC current generated by a load from transmittinginto the input voltage rail of the load voltage controller. It is anadvantage of the present invention that the cost of the implementationis lower and the size is smaller than that used by existing voltagecontrollers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] An example of the present invention will now be described by wayof example only with reference to the following drawings, in which:

[0015]FIG. 1 is a graph illustrating a typical motor current waveform;

[0016]FIG. 2 is a schematic diagram of a low frequency filter forattenuating a motor's AC current (prior art);

[0017]FIG. 3 is a schematic diagram of an active filter for attenuatinga motor's AC current (prior art);

[0018]FIG. 4a is a schematic diagram of a voltage control circuit inaccordance with an embodiment of the invention.

[0019]FIG. 4b is a more detailed schematic diagram one implementation ofthe voltage control circuit illustrated in FIG. 4a.

[0020]FIG. 5a is a graph illustrating the input voltage V_(sense);

[0021]FIG. 5b is a graph illustrating the average input voltage V_(avg);

[0022]FIG. 5c is a graph illustrating the input current I_(sense);

[0023]FIG. 6a is a graph illustrating the fan current;

[0024]FIG. 6b is a graph illustrating the output voltage V_(out); and

[0025]FIG. 6c is a graph illustrating the error voltage V_(error).

DETAILED DESCRIPTION OF THE INVENTION

[0026] For convenience, like numerals in the description refer to likestructures in the drawings. The invention provides a method forcontrolling low frequency noise currents drawn from a DC supply by loadcircuits that require substantial AC energy at low frequencies. In atelecommunication embodiment, frequencies in the voice band require verylarge inductors and capacitors to form a conventional filter withadequate attenuation. This type of requirement arises when loads drawingpulses of current at low frequencies are connected to the battery. Acooling fan is one example of this type of load.

[0027] Referring to FIGS. 4a and 4 b, an embodiment of the inventionillustrated generally by numeral 400. A battery terminal 402 of thetelecommunication equipment (not shown) is coupled with a powerconverter 406 via a high frequency filter 404. A sense resistorR_(sense) is coupled between the high frequency filter 404 and the powerconverter 406. The power converter 406 is coupled to a low frequencygenerating load, such as a fan motor, 408. A capacitor 407 is coupled inparallel between the power converter 406 and the load 408. Generally,the capacitor 407 is a component of the power converter 406, but isshown in the drawing for illustrative purposes.

[0028] A low pass filter 410 has, as its input, a voltage V_(sense)sensed across the sense resistor R_(sense). The low pass filter 410 hasits output coupled to a power converter control circuit 412. A voltageerror amplifier 414 has, as its inputs, a voltage V_(out) sensed at theload 408 and a reference voltage V_(ref). The reference voltage V_(ref)is the desired voltage across the load. The voltage error amplifier 414has its output coupled to the power converter control circuit 412.

[0029] In the present embodiment, the power converter 406 is a buckregulator. However, in practice any one of many switching powertopologies may be chosen to provide the power conversion. Thesetopologies include buck-derived topologies such as the forward, thepush-pull, the half-bridge or the full-bridge. Furthermore, flyback andboost topologies are also suitable, as will be appreciated by a personskilled in the art.

[0030] The voltage control circuit 400 uses two control loops 416 and418 for controlling the average value of the input current I_(Q) to thepower converter. The average value of the current drawn from the batteryterminal 402 will be same as the average value of I_(Q). The first loop416 comprises the sense resistor R_(sense), the low pass filter 410, thepower converter control circuit 412, and the power converter 406. Thesecond loop 418 comprises load 408, the voltage error amplifier 414, thepower converter control circuit 412, and the power converter 406.

[0031] The operation of the voltage control circuit 400 is described asfollows. The power converter 406 includes a switch that switches on andoff at a high frequency to provide a desired average voltage at itsoutput. The switching is controlled by a control signal 420 from thepower converter control circuit 412. The greater the duty cycle of thecontrol signal 420 the longer the switch remains on, resulting in agreater average voltage at the output of the power converter 406.Conversely, the lesser the duty cycle of the control signal the shorterthe switch remains on, resulting in a lower average voltage at theoutput of the power converter 406. The average voltage at the output ofthe power converter 406 is then provided to the load 408.

[0032] As a result of the power converter switching, however, the inputcurrent I_(sense) to the power converter appears as an AC current. ThisAC component frequency, unlike that induced by the load, is a highfrequency component, and is the same as the switching frequency. Thehigh frequency component of the current is filtered by the highfrequency filter 404 to avoid adversely affecting the voltage at thebattery terminal. Implementing a high frequency filter is known in theart and does not have the problems associated with implementing a lowfrequency as described in the prior art. That is, an effective highfrequency filter is relatively cheap to implement and requires littlespace.

[0033] The first control loop 416 is described as follows. Referring toFIG. 5a the sense voltage V_(sense) sensed across the resistor R_(sense)as the power converter switches on and off is illustrated. Since voltagehas a linear relationship to current, the graph also represents a scaledversion of the input current I_(sense), which is illustrated in FIG. 5c.Referring to FIG. 5b, the average value of the sense voltage V_(sense),is illustrated. The average voltage is obtained by passing the sensevoltage V_(sense) through the low pass filter 410, and is one input tothe power converter control circuit 412.

[0034] The second control loop 418 is described as follows. The voltageV_(o) across the load is sensed and input to the voltage error amplifier414. The reference voltage V_(ref) is also input to the voltage erroramplifier 414. The voltage error amplifier 414 determines and amplifiesthe difference between its two input signals and outputs an errorvoltage V_(error). Error voltage V_(error) is input to the powerconverter control circuit 412. It should be noted that the bandwidth ofthe second control loop 418 is designed to be much slower than thefrequency of the AC component of the load. This is achieved by selectingthe proper resistor and/or capacitor values for the voltage erroramplifier 414. The bandwidth desired, as well as the resistor/capacitorvalues to achieving such a limitation will be apparent to a personskilled in the art in accordance with the implementation. Typically, adesign criterion is to limit the bandwidth of the voltage loop to atleast 10 times less than the frequency of AC component of the load. Inthe present embodiment, the frequency of the motor's AC current isapproximated 350 Hz. Thus, the bandwidth of the voltage loop is designedto be 20 Hz or less. As a result, the low frequency component of thevoltage V_(o), that is the voltage related to the AC load current,across the load is filtered and the error voltage V_(error) has a DCvalue without a low frequency component. The AC current from the load isthen forced to flow through the capacitor 407. Referring to FIG. 6a-c,waveforms of the load current I_(o), voltage V_(o), and error voltageV_(error) are illustrated. Referring to FIG. 6a, the low frequency loadcurrent is represented by a sinusoidal waveform. Referring to FIG. 6b,the output voltage waveform is also sinusoidal. However, the bandwidthof the error amplifier is much lower than the load current. Therefore,referring to FIG. 6c, the V_(error) waveform has DC form.

[0035] The operation of the power converter control circuit 412 isdescribed as follows. The circuit 412 receives, as its input, theaverage voltage V_(avg) of the sense voltage V_(sense) and the errorvoltage V_(error). Typically, the sense resistor R_(sense) is small soas not to use too much power. As a result, the average voltage V_(avg)is small (in the order of millivolts). Thus, the control circuit 412amplifies the average voltage V_(avg) so that it is on a comparablescale to the voltage V_(error). As previously described, the voltageV_(error) is an amplified version of the difference between the outputvoltage and the reference voltage. Typically, it is preferable tocompare the two voltages when they are in the order of volts.

[0036] The power converter control circuit 412 tracks the averagevoltage V_(avg) to the error voltage V_(error) and adjusts the pulsewidth of the control signal 420 to the power converter 406 accordingly.The ability of the power converter control circuit to track the averagevoltage V_(avg) to the error voltage V_(error) is an important factor inlimiting the amount of low frequency AC current introduced at the inputof the power converter 406. Thus, the type of circuit used for the powerconverter control circuit depends largely on how pure of a DC signal isdesired at the input. In the preferred embodiment, the power convertercontrol circuit is achieved using an average current mode controller, aswill be appreciated by a person skilled in the art. However, in actualimplementation, peak current mode control pulse width modulationcircuits can also be used in order to reduce the cost. In this case, theinput current will contain a small amount of low frequency AC current.The low frequency AC current remaining, however, falls within therequirements set by the standard for telecommunication applications.Other implementations will become apparent to a person skilled in theart.

[0037] As described above, the average voltage V_(avg) tracks the errorvoltage V_(error). This is illustrated in the following example. Thevoltage control circuit 400 is implemented in a telecommunicationenvironment to control the motor speed of a fan in an equipment cabinet.If the temperature in the cabinet rises, for whatever reason, the fanspeed needs to increase to cool the equipment cabinet. In order to theincrease the fan speed, the voltage at the input to the fan needs to beincreased. This is achieved by increasing the reference voltage. As thereference voltage increases, the voltage V_(error) increases, since thedifference between the desired voltage and the actual voltage increases.Since the average voltage V_(avg) is tracked to the voltage V_(error),it is increased. As a result, the duty cycle of the power converter isincreased, resulting in an increased voltage at the output V_(o).

[0038] Furthermore, the average value of the voltage V_(avg), asillustrated in FIG. 5b, is tracked to the voltage V_(error), which is arelatively pure DC signal. Therefore, the average value of V_(avg) willbe equal to V_(error). Consequently the average value of the inputcurrent I_(Q) over will be same. Therefore, the input current I_(Q),will contain a DC component and high frequency switching components,which can easily be filtered by a high frequency filter. It does notcontain the low frequency components related to the load current. Aspreviously described, a high frequency filter does not have the samepractical difficulties associated with it as does a low frequencyfilter.

[0039] This combination of the two control loops 416 and 418 forces thepower controller and load to appear as a voltage controlled currentsource when observed from the input power terminals and as a voltagecontrolled power source when observed from the load terminals.

[0040] Although the invention has been described with reference tocertain specific embodiments, various modifications thereof will beapparent to those skilled in the art without departing from the spiritand scope of the invention as outlined in the claims appended hereto.

What is claimed is:
 1. In a telecommunications system, a circuit forreducing low frequency noise currents drawn from a DC power supply by aload circuit using AC power at low frequencies, said circuit comprising:a) a power converter that converts an input voltage to a desired outputvoltage by repeatedly switching said input voltage on and off; b) afirst sense circuit that senses an input signal to said power converter;c) a second sense circuit that senses a load signal at said loadcircuit; d) a comparator, coupled to said second sense circuit, thatdetermines and provides as its output a difference signal representativeof a difference between said load signal at said load circuit and apredefined reference voltage, wherein said comparator has a limitedbandwidth as compared to an AC load requirement; e) a power convertercontrol circuit, having its inputs coupled to said first sense circuitand said comparator output, and its output coupled to said powerconverter, that controls a duty cycle of switching of said powerconverter in accordance with a relationship between said input signaland said difference signal of said comparator.
 2. A circuit as definedin claim 1, wherein said first sense circuit further includes a low passfilter that provides an average value of said input signal.
 3. A circuitas defined in claim 2, further comprising an input amplifier thatamplifies said input signal.
 4. A circuit as defined in claim 3, whereinsaid comparator further includes a difference amplifier that amplifiessaid difference.
 5. A circuit as defined in claim 4, wherein said inputamplifier and said difference amplifier amplify said input signal andsaid difference, respectively, to a common order of magnitude.
 6. Acircuit as defined in claim 1, further comprising a high frequencyfilter, coupled between said DC power supply and said power converter,that reduces a high frequency noise component introduced by switching ofsaid power converter.
 7. A circuit as defined in claim 1, wherein saidload circuit comprises a fan that cools said telecommunications system.8. In a telecommunications system, a circuit for reducing low frequencynoise currents drawn from a DC power supply by a load circuit using ACpower at low frequencies, said circuit comprising: a) power convertermeans for converting an input voltage to a desired output voltage byrepeatedly switching said input voltage on and off; b) first sensecircuit means for sensing an input signal to said power converter means;c) second sense circuit means for sensing a load signal at said loadcircuit; d) comparator means, coupled to said second sense circuitmeans, for determining and providing as its output a difference signalrepresentative of a difference between said load signal at said loadcircuit and a predefined reference voltage, wherein said comparatormeans has a limited bandwidth as compared to an AC load requirement; e)power converter control circuit means, having its inputs coupled to saidfirst sense circuit and said comparator output, and its output coupledto said power converter, for controlling a duty cycle of switching ofsaid power converter means in accordance with a relationship betweensaid input signal and said difference signal of said comparator means.9. A circuit as defined in claim 8, wherein said first sense circuitmeans further includes low pass filter means for providing an averagevalue of said input signal.
 10. A circuit as defined in claim 9, furthercomprising an input amplifier means for amplifying said input signal.11. A circuit as defined in claim 10, wherein said comparator meansfurther includes difference amplifier means for amplifying saiddifference.
 12. A circuit as defined in claim 11, wherein said inputamplifier means and said difference amplifier means amplify said inputsignal and said difference, respectively, to a common order ofmagnitude.
 13. A circuit as defined in claim 8, further comprising highfrequency filter means, coupled between said DC power supply and saidpower converter means, for reducing a high frequency noise componentintroduced by switching of said power converter means.
 14. A circuit asdefined in claim 8, wherein said load circuit comprises fan means forcooling said telecommunications system.